Can anyone recommend any good Japanese dharma books? I got to a relatively high level in Japanese a few years ago but haven't used it in a while so my skills have deteriorated. I wouldn't be that interested in sutra translations since I'd rather use the classical Chinese translations to improve my Chinese but are there any good books on abhidharma in Japanese for example?

Academic articles (search for articles with .pdfs and you can download all of them). There is no shortage of articles on Buddhist related subjects.

If you find an author you like, look at their publications. Chances are they might have a work or two. Be aware that academic works in Japan cost an arm and a leg. US$300 and up for a single monograph is the usual price.